Three-dimensional oxygen isotope imaging of convective fluid flow around the Big Bonanza, Comstock lode mining district, Nevada

Oxygen isotope analyses of propylitized andesites from the Con Virginia and California mines allow construction of a detailed, three-dimensional image of the isotopic surfaces produced by the convective fluid flows that depos ited the famous Big Bonanza orebody. On a set of intersecting maps and sect ions, the delta(18)O isopleths clearly show the intricate and conformable r elationship of the orebody to a deep, similar to 500 m gyre of meteoric-hyd rothermal fluid that circulated along and above the Comstock fault, near th e contact of the Davidson Cranodiorite. The core of this gyre (delta(18)O = 0 to 3.8 parts per thousand) encompasses the bonanza and is almost totally surrounded by rocks having much lower delta(18)O values (-1.0 to -4.4 part s per thousand). This deep gyre may represent a convective longitudinal rol l superimposed on a large unicellular meteoric-hydrothermal system, produci ng a complex flow field with both radial and longitudinal components that i s consistent with experimentally observed patterns of fluid convection in p ermeable media.